The present invention relates to a gas sensor, a gas sensor system using the same, and a method of manufacturing the gas sensor.
Resistance-type sensors are known for measuring the concentration of a combustible gas component such as hydrocarbon (HC) or CO contained in the exhaust gas of an automobile or the like. For example, an oxide semiconductor (n type) such as SnO2 or the like is used as a sensing element for measuring the concentration of a combustible gas component such as HC or CO. Specifically, oxygen in the atmosphere adsorbs onto the sensing element through an effect induced by negative charges. When the atmosphere contains a combustible gas component such as HC or CO, the combustible gas component undergoes a combustion reaction with the adsorbed oxygen, thereby causing oxygen to be desorbed from the sensing element. Because a change in electric resistance of the sensing element associated with the oxygen desorption depends on the combustible gas component concentration of the atmosphere, the combustible gas component concentration of the atmosphere can be obtained by measuring the change in electric resistance. However, such a resistance-type sensor has a drawback in that an output from the sensing element formed of an oxide semiconductor is likely to vary depending on the concentration of oxygen or water vapor contained in the exhaust gas. Accordingly, even when the combustible gas component concentration remains unchanged, the sensor output value indicative of the combustible gas component concentration varies depending on, for example, the oxygen concentration of the exhaust gas.
In order to solve the above problem, an apparatus for measuring a combustible gas component concentration having the following structure is disclosed in Japanese Patent Application Laid-Open No. 8-247995. In this apparatus, the sensing element has two processing zones. An exhaust gas is introduced into a first processing zone via a first diffusion-controlling means. Oxygen is pumped out from the first processing zone by means of a first oxygen pumping element so as to reduce the oxygen concentration of the first processing zone to a low value at which combustible gas components are not substantially burned. Next, the gas having the thus-reduced oxygen concentration is introduced into a second processing zone via a second diffusion-controlling means. Oxygen is pumped into the second processing zone by means of a second oxygen pumping element so as to burn the combustible gas component. The combustible gas component concentration is determined based on the value of current flowing through or voltage developed across the second oxygen pumping element.
However, in the apparatus disclosed in the above-described patent publication, the second oxygen pumping element is operated such that the oxygen concentration of the second processing zone falls within a certain constant range. Accordingly, in addition to the second oxygen pumping element, the use of an element (for example, an oxygen concentration cell element) for measuring the oxygen concentration of the second processing zone is substantially unavoidable. Accordingly, the number of required elements for the second processing zone increases, thereby resulting in a complicated sensor structure and an increase in manufacturing cost.
Also, in the above disclosed apparatus, the oxygen concentration of the exhaust gas introduced into the first processing zone is reduced by means of the first oxygen pumping element to xe2x80x9ca low value at which a combustible gas component is not substantially burnedxe2x80x9d. According to this publication, the low value is not higher than 10xe2x88x9214 atm, preferably not higher than 10xe2x88x9216 atm, and is normally about 10xe2x88x9220 atm. However, when the oxygen concentration of the first processing zone is set at such a low value, the following problem arises related to accuracy in measuring the combustible gas component concentration.
Specifically, an exhaust gas generally contains a fair amount of water vapor in addition to combustible gas components such as hydrocarbon, carbon monoxide and hydrogen. Generally, the amount of water vapor varies according to operating conditions of an internal combustion engine. According to studies conducted by the present inventors, when the oxygen concentration of such an exhaust gas is reduced to the above-mentioned value, a portion of the water vapor is decomposed into hydrogen and oxygen. The thus-generated oxygen is pumped out from the first processing zone by means of the first oxygen pumping element, whereas the thus-generated hydrogen is not pumped out, but introduced into the second processing zone where the hydrogen induces combustion. When the gas to be measured (also referred to as an object gas) contains a combustible gas component composed primarily of hydrocarbon, the combustion of hydrogen generated by the decomposition of water vapor significantly affects accuracy in measuring the hydrocarbon concentration. Notably, the measurement examples disclosed in the above patent publication were all conducted under conditions where the water vapor concentration of the object gas was constant, and did not take into account the influence of a variation in water vapor concentration when measuring a combustible gas component concentration.
As disclosed in the above patent publication, a proton pump may be additionally used in order to pump out the thus-generated hydrogen from the first processing zone, so that only HC is selectively burned to thereby improve measurement accuracy. However, this method merely employs the proton pump as a means of last resort for coping with hydrogen generation associated with decomposition of water vapor. Addition of the proton pump makes the sensor structure and sensor control mechanism complex, causing an increase in apparatus cost. Furthermore, residual hydrogen which the proton pump has failed to pump out may induce a measurement error.
Also, the following problem is encountered. With the recent tendency of tightening exhaust gas regulations for air pollution control, internal combustion engines such as gasoline engines, diesel engines and the like engines tend to shift to lean-burn operation in order to suppress generation of HC associated with incomplete combustion. An exhaust gas produced under lean-burn conditions has an oxygen concentration higher than that produced under stoichiometric or rich conditions. When the above-described conventional apparatus is applied to such an exhaust gas, an oxygen pumping element is significantly burdened in order to reduce the oxygen concentration to the above-mentioned low value. As a result, the service life of the oxygen pumping element is shortened. Furthermore, since the operating power of the oxygen pumping element must be increased, a high output peripheral control circuit must also be used which in turn increases the apparatus cost.
It is therefore a first object of the present invention to provide a gas sensor and a method of manufacturing a gas sensor, having a simple structure and capable of accurately measuring the combustible gas component concentration of a measurement gas, such as an exhaust gas, despite variations in the oxygen concentration of the, measurement gas and despite variations in element temperature, as well as to provide a gas sensor system using the gas sensor. A second object of the present invention is to provide a gas sensor and a method of manufacturing the gas sensor in which accuracy in measuring a combustible gas component concentration is less susceptible to decomposition of water vapor and which is suitably applicable to lean-burn conditions, as well as to provide a gas sensor system using the gas sensor.
The above objects of the present invention have been achieved by providing a gas sensor having the following characteristic features.
(1) First processing space: A first processing space is provided which is isolated from the surrounding environment. A measurement gas is introduced into the first processing space via a first gas passage.
(2) Second processing space: A second processing space is provided which is isolated from the surrounding environment. A gas contained in the first processing space is introduced into the second processing space via a second gas passage.
(3) Oxygen concentration detection element: Adapted to measure the oxygen concentration of a gas contained in the first processing space.
(4) Oxygen pumping element: Formed of an oxygen-ion conductive solid electrolyte. Electrodes are formed on opposing surfaces of the oxygen pumping element. The oxygen pumping element pumps out oxygen from the first processing space or pumps oxygen into the first processing space so as to reduce the oxygen concentration of the measurement gas introduced into the first processing space and measured by the first oxygen concentration detection element to a predetermined level.
(5) Oxidation catalyst element: Adapted to accelerate combustion of a combustible gas component contained in the gas which has been introduced into the second processing space from the first processing space via the second gas passage.
(6) Combustible gas component concentration detection element: Formed of an oxygen-ion conductive solid electrolyte. Electrodes are formed on opposing surfaces of the combustible gas component concentration detection element. One of the electrodes is arranged so as to be exposed to the second processing space. A constant voltage is applied to the combustible gas component concentration detection element via the electrodes. The combustible gas component concentration detection element has an output current which varies according to the amount of oxygen consumed by combustion of the combustible gas component, to thereby provide information regarding the concentration of the combustible gas component of the measurement gas.
The above objects of the present invention have also been achieved by providing a gas sensor system having the following constituent features.
(A) Gas sensor: Configured to have the following characteristic features.
(1) First processing space: A first processing space is provided which is isolated from the surrounding environment. A measurement gas is introduced into the first processing space via a first gas passage.
(2) Second processing space: A second processing space is provided which is isolated from the surrounding environment. A gas contained in the first processing space is introduced into the second processing space via a second gas passage.
(3) Oxygen concentration detection element: Adapted to measure the oxygen concentration of a gas contained in the first processing space.
(4) Oxygen pumping element: Formed of an oxygen-ion conductive solid electrolyte. Electrodes are formed on opposing surfaces of the oxygen pumping element. The oxygen pumping element pumps out oxygen from the first processing space or pumps oxygen into the first processing space.
(5) Oxidation catalyst element: Adapted to accelerate combustion of a combustible gas component contained in a gas having an oxygen concentration which has been adjusted by the oxygen pumping element and then introduced into the second processing space from the first processing space via a second gas passage.
(6) Combustible gas component concentration detection element: Formed of an oxygen-ion conductive solid electrolyte. Electrodes are formed on opposing surfaces of the combustible gas component concentration detection element. One of the electrodes is arranged so as to be exposed to the second processing space. A constant voltage is applied to the combustible gas component concentration detection element via the electrodes. The combustible gas component concentration detection element has an output current which varies according to the amount of oxygen consumed by combustion of the combustible gas component, to thereby provide information regarding the concentration of the combustible gas component of the measurement gas.
(B) Oxygen pumping operation control means: Adapted to control the oxygen pumping element so as to reduce the oxygen concentration of the measurement gas introduced into the first processing space and measured by the oxygen concentration detection element to a predetermined value.
(C) Voltage source: Adapted to apply a constant voltage to the combustible gas component concentration detection element.
The gas sensor and the gas sensor system of the present invention can measure a combustible gas component selected singly or in combination from the group consisting of, for example, hydrocarbon (HC), carbon monoxide and hydrogen.
In the configuration described above, the oxygen concentration of a measurement gas contained in the first processing space is adjusted to a predetermined value by operation of the oxygen pumping element. The thus-treated gas is introduced into the second processing space, where a combustible gas component undergoes combustion through the aid of the oxidation catalyst. The combustible gas component concentration detection element, to which a small constant voltage is applied, has an output current which varies according to the oxygen consumption due to combustion of the combustible gas component. Thus, based on the output current, information regarding the combustible gas component concentration of the original measurement gas is obtained. Because the oxygen concentration of a measurement gas detected by the oxygen concentration detection element is adjusted to a predetermined level before the combustible gas component concentration of the gas is measured, the output from the combustible gas component concentration detection element is less affected by the original oxygen concentration of the measurement gas. Thus, the relationship between the output current of the combustible gas component concentration detection element and the combustible gas component concentration of the measurement gas exhibits good linearity. Furthermore, in the conventional sensor disclosed in the above publication, at least two elements, namely the oxygen pumping element and the oxygen concentration measuring element, must be provided for the second processing zone (corresponding to the second processing space in the present invention). By contrast, in the configuration of the present invention, the combustible gas component concentration detection element suffices in and of itself, thereby simplifying sensor structure. Thus, the above-mentioned configuration achieves the first object of the present invention.
The above-described small constant voltage can be applied to the combustible gas component concentration detection element in a polarity which generates in the combustible gas component concentration detection element an oxygen pumping current which flows in a direction causing oxygen to be pumped out from the second processing space. In this case, the applied voltage is preferably adjusted such that a partial pressure of oxygen in the gas introduced into the second processing space does not cause a substantial decomposition of nitrogen oxides contained in the gas. This prevents a reduction in accuracy in measuring a combustible gas component concentration which would otherwise result from oxygen generated from the decomposition of NOx.
In the above-described gas sensor of the present invention, the oxygen pumping element can adjust the oxygen concentration of the measurement gas introduced into the first processing space and detected by the oxygen concentration detection element within a range such that water vapor contained in the measurement gas is not substantially decomposed. In this case, the oxygen pumping operation control means of the gas sensor system controls the operation of the oxygen pumping element such that the oxygen concentration of the measurement gas introduced into the first processing space and measured by the oxygen concentration detection element is adjusted within a range such that water vapor contained in the measurement gas is not substantially decomposed. Namely, within a range which does not substantially initiate a reaction of decomposing water vapor contained in the measurement gas.
By substantially suppressing hydrogen generation associated with the decomposition of water vapor by oxygen concentration adjustment, a loss in accuracy in measuring a combustible gas component concentration can be prevented which would otherwise result from combustion of the generated hydrogen. Also, the gas sensor and the gas sensor system of the present invention exhibit excellent selectivity toward HC, particularly methane, and thus can measure methane concentration more accurately than do conventional gas sensors. Thus, the second object of the present invention has been achieved.
When a measurement gas contains carbon dioxide and the carbon dioxide is decomposed, carbon monoxide which is a combustible gas component is generated as in the case of water vapor which generates hydrogen. Combustion of the thus-generated carbon monoxide may lower the accuracy in detecting the combustible gas component. In this case, the first oxygen pumping element preferably adjusts the oxygen concentration of the measurement gas introduced into the first processing space and detected by the oxygen concentration detection sensor within a range such that carbon dioxide is not substantially decomposed. Because the oxygen concentration at which decomposition of carbon dioxide occurs is generally lower than the oxygen concentration at which decomposition of water vapor occurs, the decomposition of carbon dioxide is concurrently prevented by employing an oxygen concentration which prevents decomposition of the water vapor.
The oxygen pumping element can be configured to adjust the oxygen concentration of the measurement gas introduced into the first processing space within a range of 10xe2x88x9212 atm to 10xe2x88x926 atm. In this case, the oxygen pumping operation control means of the gas sensor system controls the operation of the oxygen pumping element such that the oxygen concentration of the measurement gas introduced into the first processing space and measured by the oxygen concentration detection element is adjusted within a range of 10xe2x88x9212 atm to 10xe2x88x927 atm.
In the above-described configuration, the oxygen concentration of the first processing space achieved by operation of the oxygen pumping element is adjusted so as to fall within the above range, thereby suppressing the decomposition of water vapor and thus improving the sensing accuracy of the gas sensor or the gas sensor system. Because the oxygen concentration to be achieved by adjustment is far higher than the conventionally required oxygen concentration of 10xe2x88x9220 atm to 10xe2x88x9214 atm, the oxygen pumping element assumes a smaller burden even when measuring, for example, under lean-burn conditions. Thus, the service life of the oxygen pumping element is extended. Also, the power needed to operate the oxygen pumping element is not very high, and a control circuit and other peripheral devices can be provided at low cost. Also, in this case, the oxygen pumping element (or the oxygen pumping control means) is preferably configured such that the oxygen concentration of a measurement gas introduced into the first processing space and measured by the oxygen concentration detection element is adjusted to within a range such that water vapor contained in the measurement gas is not substantially decomposed. In the gas sensor and the gas sensor system described above, in order to further effectively prevent the decomposition of water vapor, the oxygen pumping element is preferably operated such that the oxygen concentration of the first processing space is adjusted to a value at which a portion of the combustible gas component is burned in the first processing space while the first electrode serves as an oxidation catalyst.
Furthermore, when the detection selectivity toward hydrocarbon (especially, methane or the like having a relatively low combustion activity) needs to be improved, the oxygen concentration within the first processing space as measured by the oxygen concentration detection element is preferably adjusted to fall within a range such that a component (e.g., carbon monoxide, hydrogen, ammonia) having a higher combustion activity than the hydrocarbon to be detected is burned more readily than hydrocarbon. This adjustment improves the detection selectivity toward hydrocarbon (e.g., methane). The oxygen concentration range varies depending on the combustion catalytic activity of the first and third electrodes, described below, toward various combustible gas components. However, the oxygen concentration range is generally 10xe2x88x9212-10xe2x88x926 atm, preferably 10xe2x88x9211-10xe2x88x929 atm.
When the oxygen concentration of a measurement gas introduced into the first processing space becomes less than 10xe2x88x9212 atm, the decomposition of water vapor, if present, becomes conspicuous. As a result, hydrogen generated by the decomposition of water vapor may significantly impair accuracy in measuring a combustible gas component concentration. By contrast, when the oxygen concentration of the first processing space is in excess of 10xe2x88x926 atm, combustion of a combustible gas component becomes conspicuous in the first processing space. Accordingly, the combustible gas component concentration of a gas introduced into the second processing space becomes small with a potential failure to attain a predetermined measurement accuracy. More preferably, the oxygen concentration of the first processing space is adjusted to a value of 10xe2x88x9211 atm to 10xe2x88x929 atm.
For example, when the gas sensor is set at a working temperature of 650xc2x0 C. to 700xc2x0 C. and the water vapor concentration of a measurement gas varies within a range of about 5% to 15%, oxygen that maintains equilibrium with water vapor and hydrogen has a minimum partial pressure of about 10xe2x88x9212 atm. When the partial pressure of oxygen drops below the minimum value, decomposition of water vapor progresses, thereby affecting accuracy in measuring a combustible gas component concentration. Therefore, in this case, the oxygen concentration of the first processing space is preferably set to a value greater than the above minimum partial pressure of oxygen.
As used herein, unless specifically described otherwise, the oxygen concentration within the first processing space means the oxygen concentration measured by the oxygen concentration detection element. For example, when a part of a combustible gas component contained in a measurement gas burns and consumes oxygen, the oxygen concentration measured by the oxygen concentration detection element is not necessarily equal to the oxygen concentration before oxygen is consumed due to combustion. Also, the oxygen concentration may vary at locations within the first processing space due to the presence of a porous electrode that is disposed to face the first processing space and catalyzes combustion of a combustible gas component, or due to oxygen pumping of the oxygen pumping element. In this case as well, the oxygen concentration detection element is considered to represent the oxygen concentration within the first processing space. When the oxygen concentration of a measurement gas introduced into the first processing space is relatively high, the first oxygen pump operates to mainly pump oxygen out from the first processing space in order to cause the oxygen concentration detected by the oxygen concentration detection element to fall within the range of 10xe2x88x9212-10xe2x88x926 atm. By contrast, the first oxygen pump operates to pump oxygen into the first processing space when an increased amount of a combustible gas component (e.g., carbon monoxide, hydrogen, ammonia) burns while the first electrode, described below, is used as a catalyst for combustion, and oxygen consumption due to combustion proceeds.
In the gas sensor (and the gas sensor system) described above, at least either the first gas passage for introducing a measurement gas into the first processing space or the second gas passage for establishing communication between the first processing space and the second processing space may be configured as a diffusion-controlling passage for permitting gas flow at a constant diffusion resistance. This feature suppresses the compositional variation of a gas introduced into the first or second processing space to a small degree for a constant period of time determined by the diffusion resistance of the passage even when the composition of the measurement gas varies. Thus, accuracy in measuring the concentration of a combustible gas component concentration can be improved. Specifically, the diffusion-controlling passage may assume the form of small holes or slits or may be formed of any of various throttling mechanisms or porous metals or ceramics having communicating pores formed therein.
In the gas sensor and the gas sensor system described above, the oxygen concentration detection element may be an oxygen concentration cell element. The oxygen concentration cell element is formed of an oxygen-ion conductive solid electrolyte having electrodes formed on both (opposing) surfaces thereof. One of the electrodes is disposed so as to be exposed to the first processing space. In this case, the electrode exposed to the first processing space is defined as a first electrode, and the electrode of the combustible gas component concentration detection element exposed to the second processing space is defined as a second electrode. The first and second electrodes may each assume the form of a porous electrode having oxygen molecule desorption capability. The second electrode can be adapted to serve as the above-described oxidation catalyst section having oxidation-related catalytic activity toward a combustible gas component contained in the measurement gas.
In the above configuration, the first electrode has an oxidation-related catalytic activity that is lower than that of the second electrode. Thus, at least a portion of a residual combustible gas component which has not been burned in the first processing space can be reliably burned in the second processing space, thereby improving sensor sensitivity. Furthermore, because the electrode (second electrode) of the combustible gas component concentration detection element exposed to the second processing space also serves as an oxidation catalyst section, the structure of the gas sensor or the gas sensor system is further simplified.
A porous metal layer other than the second electrode may be formed on a wall portion which, together with other portions, defines the second processing space, in such manner as to be exposed to the second processing space. The porous metal layer serves as the oxidation catalyst section exhibiting oxidation-related catalytic activity toward a combustible gas component contained in the measurement gas. This feature improves the efficiency of combustion of a combustible gas component in the second processing space and thus improves the sensitivity of the gas sensor. Furthermore, the porous metal layer may be used as a porous electrode for another application (for example, as a porous electrode of an oxygen pumping element or oxygen concentration cell element other than those of the present invention).
When the gas component to be measured is CO or HC, an electrode having higher oxidation-related catalytic activity may be formed of Pt, Pd, Rh, a Pt alloy, a Pd alloy, an Rh alloy, a Ptxe2x80x94Rh alloy, an Rhxe2x80x94Pd alloy, a Pdxe2x80x94Ag alloy, or a like metal (hereinafter these metals are referred to as metals of a high-activity metal group). An electrode having lower oxidation-related catalytic activity may be formed of Au, Ni, Ag, an Au alloy, an Ni alloy, an Ag alloy, a Ptxe2x80x94Pd alloy, a Ptxe2x80x94Au alloy, a Ptxe2x80x94Ni alloy, a Ptxe2x80x94Ag alloy, an Auxe2x80x94Pd alloy, an Auxe2x80x94Pd alloy, or a like metal (hereinafter these metals are referred to as metals of a low-activity metal group). When a ZrO2 solid electrolyte, described below, is used as an oxygen-ion conductive solid electrolyte constituting a main portion of the oxygen pumping element, the oxygen concentration cell element, or the combustible gas component concentration detection element, a metal of the low-activity metal group is preferably selected such that it can be fired with the ZrO2 solid electrolyte (firing temperature: 1450xc2x0 C to 1500xc2x0 C.), in view of improving in sensor-manufacturing efficiency. For example, when a Ptxe2x80x94Au alloy is used, the Au contents thereof may be 0.1% to 3% by weight. When the Au content is less than 0.1% by weight, an electrode formed of the alloy may have an excessively high oxidation-related catalytic activity. By contrast, when Au is added in excess of 3% by weight, the amount of Au volatilizing from the alloy during firing increases, thus increasing the amount of wasted Au.
In the gas sensor of the present invention, a more preferable result is obtained by employing an electrode having the following structure. Specifically, the oxygen pumping element is formed of an oxygen-ion conductive solid electrolyte having electrodes formed on both surfaces thereof, and one of the electrodes (hereinafter referred to as the xe2x80x9cthird electrodexe2x80x9d) is disposed so as to be exposed to the first processing space. When the component to be detected is CO or HC, the third electrode is composed of two layers, namely, a porous main electrode layer and a porous surface electrode layer. The porous main electrode layer is made of Ptxe2x80x94Au alloy (Au content: 1 wt. % or less) or Pt. The porous surface electrode layer covers the main electrode layer to thereby form a surface layer portion of the third electrode. The surface electrode layer is made of a material selected from the group consisting of a metal containing Au or Ag as a main component, a Ptxe2x80x94Au alloy, an Auxe2x80x94Pd alloy, a Ptxe2x80x94Ag alloy and a Ptxe2x80x94Ni alloy (hereinafter collectively referred to as xe2x80x9cinactive metalxe2x80x9d). The third electrode has a lower oxidation-related catalytic activity toward the combustible gas component than does the second electrode. As used herein, the term xe2x80x9cX-Y alloyxe2x80x9d means an alloy in which a metal component having the highest content by weight is X, and a metal component having the second highest content by weight is Y. The alloy may be an X-Y binary system alloy or a higher-order system alloy containing X, Y and other alloy components.
Materials for the electrodes of the oxygen concentration cell element or the oxygen pumping element must have a sufficient catalytic activity for desorption and recombination of oxygen molecules. Pt single metal, for example, is an excellent material in this point. However, if this material is used for the electrode exposed to the first processing space, the material has an extremely high combustion catalytic activity toward a combustion gas component. Therefore, the catalytic activity must be decreased slightly. For example, as conventionally practiced, Au, whose combustion catalytic activity is low, is mixed with Pt in an amount of up to about 20 wt. %, thereby forming a Ptxe2x80x94Au alloy. However, when the Au content increases, a considerable decrease in activity for desorbing oxygen molecules occurs concurrently with a decrease in the combustion catalytic activity toward a combustible gas component. Therefore, these two catalytic activities are difficult to balance.
This problem can be solved by employing the above-described multilayer electrode, in which the surface of the porious main electrode layer formed of a Ptxe2x80x94Au alloy or Pt having a high activity for desorbing oxygen molecules is covered with the porous surface electrode layer formed of an inactive metal having a low combustion catalytic activity toward a combustible gas component. This structure allows for a convenient adjustment to decrease the combustion catalytic activity toward a combustible gas component to the extent possible, while maintaining a sufficient level of oxygen molecule desorption activity.
In the present invention, the surface electrode layer is preferably formed of an Au-containing porous metal that has a considerably low combustion catalytic activity toward CO or HC and some degree of catalytic activity for desorption and recombination of oxygen molecules. Alternatively, a porous metal containing Ag as a main component, a porous Ptxe2x80x94Au alloy (Au content: 5 wt. % or more), a porous Pt-Pb alloy (Pb content: 1 wt. % or more), a porous Ptxe2x80x94Ag alloy (Ag content: 1 wt. % or more), a porous Ptxe2x80x94Ni alloy (Ni content: 1 wt. % or more), and the like may be used.
The surface electrode layer and the main electrode layer may be arranged such that these layers are into indirect contact with each other via one or more other layers. However, the use of a two-layer structure comprising the main electrode layer and the surface electrode layer simplifies the manufacturing process. In this case, when the surface electrode layer is formed of an Au-containing porous metal that contains Au as a main component, the remarkable effect of suppressing the combustion catalytic activity toward a combustible gas component can be obtained, while a sufficient level of oxygen molecule desorption activity is also maintained.
The above-described multilayer electrode is advantageously employed as the third electrode of the oxygen pump which does not require a sharp response to oxygen concentration. The above-described multilayer electrode can be used as the first electrode of the oxygen concentration cell element. However, in order to further improve accuracy in detecting the oxygen concentration within the first processing space using the oxygen concentration cell element, the first electrode is preferably formed of Pt, a Ptxe2x80x94Au alloy or a Ptxe2x80x94Ag alloy. In this case, because combustion of a combustible gas component that is caused by the first electrode within the first processing space can be suppressed by making the area of the first electrode smaller than that of the third electrode, the loss caused by combustion of the combustible gas component within the first processing space can be decreased, so that the sensor sensitivity can be further increased.
When a Ptxe2x80x94Au alloy or a Ptxe2x80x94Ag alloy is used as the first electrode, Au or Ag is added in order to suppress the combustion catalytic activity toward CO or HC. In this case, when the Au or Ag content exceeds 1 wt. %, the oxygen molecule desorption activity decreases excessively, resulting in a deteriorated oxygen concentration detection performance. By contrast, when the Au or Ag content is less than 1 wt. %, almost no effect of suppressing the combustion catalytic activity is expected. Au and Ag may be added together to Pt such that their total content does not exceed 1 wt. %.
When detection selectivity for hydrocarbon among various combustible gas components must be improved, components having a higher combustion activity than hydrocarbon are preferably burned more readily than the hydrocarbon to be detected. In this case, as described above, the oxygen concentration within the first processing space as measured by the oxygen concentration detection element is adjusted. Furthermore, the combustion catalytic activity of first electrode or the third electrode exposed to the first processing space and the temperature within the first processing space are important factors in improving the measurement selectivity. When the third electrode is formed of the above-described multilayer electrode having a relatively low combustion catalytic activity and the first electrode is formed of Pt or a Pt alloy having a high combustion catalytic activity, a hydrocarbon component (e.g., methane) having a slightly low combustion activity does not burn much, while components such as carbon monoxide, hydrogen and ammonia which have a higher combustion activity readily burn on the first electrode. As a result, an environment convenient for selective measurement of hydrocarbon components is created. When the temperature within the first processing space increases, the combustion reaction proceeds easily, and the difference in combustion catalytic activity between electrodes made of different materials is not so apparent. This is disadvantageous for the selective measurement of hydrocarbon components. However, when the third electrode has the above-described multilayer structure, a considerably large difference in catalytic activity between the third electrode and the first electrode formed of Pt or the like is produced even at considerably high temperatures (e.g. 700-800xc2x0 C.), so that selective detection of hydrocarbon components can be performed effectively.
When the third electrode is formed into the above-described multilayer structure, the gas sensor of the present invention can be manufactured in accordance with a method comprising the following steps.
(1) a substrate electrode layer forming step which comprises forming a substrate electrode pattern containing an unfired layer of material powder for the main electrode layer of the third electrode (hereinafter referred to as the xe2x80x9cunfired main electrode layerxe2x80x9d) on an unfired compact of the oxygen-ion conductive solid electrolyte layer constituting said first oxygen pumping element (hereinafter referred to as the xe2x80x9cunfired solid electrolyte compactxe2x80x9d), and integrally firing the unfired main electrode layer with the unfired solid electrolyte compact at a first temperature to form on the oxygen-ion conductive solid electrolyte layer a substrate electrode layer containing the main electrode layer; and
(2) a surface electrode layer forming step which comprises forming a layer of material powder for the surface electrode layer on the substrate electrode layer, and subjecting to secondary firing at a second temperature lower than the first temperature to thereby form the surface electrode layer. The layer of material power may be formed, for example, by applying a paste of the material powder onto the main electrode layer.
Because the substrate electrode layer containing the main electrode layer is formed of a high-melting point metal such as Pt or a Ptxe2x80x94Au or Ptxe2x80x94Ag alloy having the above-described composition, the substrate electrode layer can be fired concurrently with a solid electrolyte ceramic, such as zirconia, that constitutes the main portion of each element. However, when the surface electrode layer is formed of an Au-containing metal, which has a low melting point, maintaining the porous state of the substrate electrode layer becomes difficult when it is fired together with a solid electrolyte ceramic. In addition, Au diffuses into the substrate electrode layer, and therefore it becomes difficult to achieve the effect of suppressing the combustion catalytic activity. In order to solve this problem, the above-described process can be employed in which the surface electrode layer is subjected to secondary firing at a temperature lower than that used for integrally firing the substrate electrode layer and the solid electrolyte layer. This is to bond the surface electrode layer to the substrate electrode layer by baking. Thus, a multilayer electrode having the desired performance is obtained.
The components (e.g., Au) of the surface electrode layer may diffuse into the main electrode layer during the secondary firing or when the sensor is used at high temperature. For example, even if the main electrode layer is substantially formed of Pt, Au may diffuse from the surface electrode layer into the main electrode layer so that Au constituting the main electrode layer is converted into a Ptxe2x80x94Au alloy. If diffusion of the material of the surface electrode layer into the main electrode layer proceeds excessively, the thickness of the surface electrode layer becomes insufficient, or in an extreme case, the surface electrode layer disappears. For example, when the surface electrode layer is desirably formed mainly of Au and the main electrode layer is desirably formed mainly of Pt, the temperature for secondary firing is preferably set to about 800-1050xc2x0 C. in order to prevent excessive diffusion of Au into the main electrode layer. When the secondary firing temperature is less than 800xc2x0 C., firing of the surface electrode layer becomes insufficient with the possibility of delamination of the surface electrode layer due to inadequate contact. By contrast, when the secondary firing temperature is greater than 1050xc2x0 C., the thickness of the surface electrode layer becomes insufficient due to diffusion of the Au component, or firing proceeds excessively such that the porous structure is lost. In this case, the oxygen permeability that the porous electrode must have becomes difficult to maintain. When Au is mixed in the constituent metal of the main electrode layer in an amount of about 3-10 wt. % (for example, 10 wt. %) from the beginning, the diffusion of Au from the surface electrode layer into the main electrode layer can be suppressed because the extent of solid solution formation of Au into Pt is relatively small (about 5 wt. %) at 800xc2x0 C. Thus, the drawbacks such as a reduction in thickness of the surface electrode layer can be effectively avoided.
In a preferred embodiment, the manufacturing method comprising the above-described secondary firing step can be performed efficiently when the gas sensor of the present invention is constructed such that a pumping cell unit including the oxygen pumping element is formed separately from a sensor cell unit including the oxygen concentration detection element, the second processing space and the combustible gas component concentration detection element; and the pumping cell unit and the sensor cell unit are joined and integrated with each other via a bonding material. In this case, the pumping cell unit is manufactured by firing such that the substrate electrode layer is formed without forming the surface electrode layer; the secondary firing is performed in order to form the surface electrode layer on the substrate electrode layer of the pumping cell unit; and the pumping cell unit is integrated with the sensor cell unit which has been separately manufactured through firing. Thus, the gas sensor is obtained. Preferably, a pump-cell-side fitting portion is formed in the pumping cell unit, and a sensor-cell-side fitting portion for engaging with the pump-cell-side fitting portion is formed in the sensor cell unit. In this case, positioning during joining can be easily performed by engaging the pump-cell-side fitting portion and the sensor-cell-side fitting portion. Thus, the manufacturing efficiency of the sensor can be improved.
For stabilizing the sensor output, the electrode (second electrode) of the combustible gas component concentration detection element exposed to the second processing space be is preferably positioned so as not to interfere with or contact the second gas passage. When the electrode is in contact with the second gas passage, combustion of a combustible gas component may be initiated before equilibrium is established between a measurement gas which is newly introduced into the second processing space from the first processing space and a gas which is already present in the second processing space. When such positional interference is avoided, the above phenomenon is less likely to occur, thereby stabilizing the sensor output.
The oxygen concentration cell element or the combustible gas component concentration detection element may be formed of an oxygen-ion conductive solid electrolyte composed mainly of ZrO2 (ZrO2 solid electrolyte). In the oxygen concentration cell element formed of a ZrO2 solid electrolyte one electrode is in contact with a gas to be measured, which gas contains oxygen and a combustible gas component, while the other electrode is in contact with a reference atmosphere having a constant oxygen concentration. The electromotive force of the oxygen concentration cell element varies abruptly when the gas composition falls outside a stoichiometric composition in which oxygen and a combustible gas component are present in a proper ratio so that they completely react with each other. When an ordinary gasoline engine or diesel engine is operated under lean-burn conditions, a measurement gas emitted from the engine contains combustible gas components in a total concentration of about 0 to 1000 ppmc (ppmc: parts per million represented with carbon equivalent). A measurement gas having such a combustible gas component concentration is introduced into the first processing space, and the oxygen concentration of the thus introduced measurement gas is adjusted to 10xe2x88x926 atm (preferably 10xe2x88x929 atm) or lower, as described above. As a result, a gas introduced into the second processing space from the first processing space has a stoichiometric composition or a composition shifted slightly toward a rich condition. Thus the output from the combustible gas component concentration detection element is increased, thereby improving the sensitivity of the gas sensor.
When the oxygen pumping element, the oxygen concentration cell element and the combustible gas component concentration detection element are formed of a ZrO2 solid electrolyte as described above, a heating element may be provided for heating the elements to a predetermined working temperature. The working temperature may be set to 650xc2x0 C. to 700xc2x0 C. When the working temperature is in excess of 700xc2x0 C., the output current value of the combustible gas component concentration detection element becomes excessively low, causing a reduction in sensitivity of the gas sensor. This is considered to occur because most of the combustible gas component, such as an HC component contained in a measurement gas, is burned in the first processing space due to the high working temperature. By contrast, when the working temperature is lower than 650xc2x0 C., the internal resistance of the oxygen pumping element increases, causing unstable operation. As a result, accuracy in measuring a combustible gas component may be reduced.
As described above, in the gas sensor and the gas sensor system of the present invention, the oxygen concentration detection element may be an oxygen concentration cell element formed of an oxygen-ion conductive solid electrolyte having electrodes formed on both surfaces thereof, wherein one electrode (the first electrode) serves as a detection electrode and is exposed the first processing space, while the other electrode serves as an oxygen reference electrode. In this case, the oxygen reference electrode may be used as an electrode of the combustible gas component concentration detection element having another electrode (second electrode) that is exposed to the second processing space. This arrangement enables the oxygen concentration cell element and the combustible gas component concentration detection element to share the oxygen reference electrode, to thereby implement a compact sensor.
More specifically, the first processing space and the second processing space may be arranged such that a partition wall formed of an oxygen-ion conductive solid electrolyte is disposed therebetween. In this case, the second gas passage is formed in the partition wall so as to establish communication between the first processing space and the second processing space. An oxygen reference electrode is embedded in the partition wall at a thicknesswise intermediate portion. The first electrode is formed on the partition wall so as to be exposed to the first processing space. The oxygen concentration cell element is formed by the first electrode, the oxygen reference electrode and a portion of the partition wall interposed between the first electrode and the oxygen reference electrode. Also, the second electrode is formed on the partition wall so as to be exposed to the second processing space. The combustible gas component concentration detection element is formed by the second electrode, the oxygen reference electrode and a portion of the partition wall interposed between the second electrode and the oxygen reference electrode. The oxygen pumping element is arranged opposite the partition wall with the first processing space disposed therebetween. This arrangement enables the oxygen concentration detection element and the combustible gas component concentration detection element to share the oxygen reference electrode, to thereby implement a compact sensor.
The oxygen reference electrode of the oxygen concentration cell element may be a self-generation type oxygen reference electrode in which a very small pumping current is applied between the detection electrode and the oxygen reference electrode in a direction such that oxygen is pumped toward the oxygen reference electrode side. As a result, a reference oxygen concentration of a predetermined level is established within the oxygen reference electrode by the pumped-in oxygen. This structure stabilizes the oxygen concentration at the oxygen reference electrode side, and allows for a more accurate measurement of the oxygen concentration.
Also, when the oxygen concentration cell element and the combustible gas component concentration detection element share the oxygen reference electrode and the oxygen reference electrode is designed to function as a self-generation type oxygen reference electrode, a current limit circuit is preferably provided to limit within a predetermined range the current flowing between the second electrode and the oxygen reference electrode. More specifically, the current limit circuit is designed to limit within a predetermined range the current flowing from the second electrode to the oxygen reference electrode.
Namely, the fact that a current flows from the second electrode to the oxygen reference electrode means that oxygen flows out from the oxygen reference electrode toward the second electrode. This is because the oxygen-ion conductive solid electrolyte is interposed between these electrodes. If the current flow is excessive, a large amount of oxygen flows out from the oxygen reference electrode, such that the oxygen reference electrode cannot maintain the required oxygen concentration. As a result, proper operation of the oxygen concentration cell element or proper control of the oxygen concentration within the first processing space becomes difficult, resulting in a decrease in the detection accuracy of the sensor. However, this drawback can be avoided by providing the above-described current limit circuit.
Furthermore, the current limit circuit may be designed to limit within a predetermined range the current flowing from the oxygen reference electrode to the second electrode. If the current flow is excessive, a large amount of oxygen flows into the oxygen reference electrode. As a result, the pressure within the oxygen reference electrode becomes excessively high such that the electrode may break. However, this problem can be avoided by providing the above-described current limit circuit.
In this case, at least either the oxygen reference electrode or the second electrode is preferably formed in or on the partition wall at a position so as not to contact the second gas passage. More preferably, both of the reference electrode and the second electrode are positioned so as not to contact the second gas passage. Positioning the second electrode in such a manner yields the aforementioned advantage. Also, positioning the oxygen reference electrode in such a manner prevents leakage of oxygen from the oxygen reference electrode through the second gas passage, thereby stabilizing the oxygen reference concentration and thus stabilizing the sensor output which is indicative of the combustible gas component concentration
In the above gas sensor, the current flowing through the oxygen pumping element of the gas sensor, i.e., the oxygen pumping current, varies according to the oxygen concentration of the measurement gas. Accordingly, the oxygen pumping current is a measure of the oxygen concentration of the measurement gas. Therefore, in the gas sensor system of the present invention, correction means may be provided for correcting the output of the combustible gas component concentration detection element based on the oxygen concentration of the measurement gas as determined by the oxygen pumping current. That is, as described above, the gas sensor system of the present invention is characterized as being less susceptible to the oxygen concentration of a measurement gas. Nevertheless, when the oxygen concentration causes variations in the output, such variations can be corrected by the correction means, thereby further improving accuracy in measuring a combustible gas concentration.
Specifically, the correction means may include storage means and correction value determination means. The storage means stores information regarding the relationship between the output current of the combustible gas component concentration detection element and the combustible gas component concentration, relative to various values of oxygen concentration (or values of oxygen pumping current). The correction value determination means determines a corrected output current (or a corresponding combustible gas component concentration) based on the output current of the combustible gas component concentration detection element and the above information. Thus, the measured combustible gas component concentration can be corrected to take into account the oxygen concentration of the measurement gas.